The present invention relates to a method for precision magnification calibration of image magnifying devices, such as optical microscopes, confocal scanning microscopes, transmission electron microscopes, scanning electron microscopes, tunneling microscopes and atomic force microscopes.
Technical advances in many scientific fields are placing demands for increasing accuracy of feature measurements made using image-magnifying instruments [micro-scopes], in the size ranges of millimeters, microns and nanometers. In particular, the need arises because of the increasing miniaturization and complexity of integrated circuits and cellular studies in the field of bioengineering.
However, accurate size measurements cannot be made with a microscope unless it has first been calibrated with a magnification reference “standard” (standard is defined as a device having a verifiable value). The reliability of the information obtained from microscope image measurements depends on the accuracy of the microscope's magnification calibration. Any error in the magnification calibration of a microscope is a component of the total size measurement error and frequently is the predominant component. The magnification scale M of the microscope is determined by the ratio. M=L′/L. L is the size of the test object (measurement reference) used for the magnification calibration; L′ is the size of the same test object realized in the microscope image (the magnified image of the object).
The methods of magnification calibration are known B. B. Martinov, “Problems of Measurements of Linear Sizes of Relief Sub Micron Structures on Raster Electronic Microscopes” preprint #501IOFAN.M, 1990, page 18, assuming use of a line-width standard (magnification reference) as the test object. Thus, it is considered that the nominal size L of the magnification reference is known with an adequate accuracy. However, obtaining the exact pitch value L′ from the microscope's image of the test object pattern appears to be a problem in this case. The fact is that a ratio between an object and its image is rather nontrivial.
In any kind of microscopy the image is only similar to the object, but is never an exact copy. In particular, there are no universal rules, according to which it would be possible to specify points on the image corresponding to object edges (the distance between which is the image size L′). This is the reason for significant errors in the value of L′ and for microscope magnification calibration errors as a whole. In B. B. Martinov's “Problems of Measurements of Linear Sizes of Relief Sub Micron Structures on Raster Electronic Microscopes” preprint #501IOFAN.M, 1990, page 18, the results of a practical application of this method are given. The calibration error of the Scanning Electron Microscope (SEM) was calculated by the authors as 2.6% in one case and as 5.1% in the other case. This is not an acceptable magnification error for a SEM used to perform accurate measurements. Such SEMs require their magnification error to be less than 0.5%.
The use of a pitch magnification reference material as a standard for magnification calibration M. T. Postek, Critical Issues in Scanning Electron Microscopes Metrology, Journ. Of Research of the National Inst. of Standards & Technol., Vol. 99, No. 5, October 1994, pp. 658–660 provides significant advantages in the precision of a microscope's magnification calibration. As the pitch reference contains several or many repeatable identical features (lines or stripes). Independent of the type or model of microscope being calibrated, these patterned lines will appear to be identical to each other. This strongly facilitates evaluation of the pitch value of such structures present in the microscope image: the distance between any equivalent points of adjacent stripe pattern features in the image can be considered as the pitch value. Such points can be established or noted by using the maxima or minima of brightness in the video signal, any repeated characteristic features on the videosignal slopes, etc. In order to implement such a method, firstly it is necessary to create and certify the indicated pitch magnification references with a known accuracy (creating a standard), there by correlating their nominal pitch size to an absolute size scale. Both of these problems are not simple. According to M. T. Postek, Critical Issues in Scanning Electron Microscopes Metrology, Journ. Of Research of the National Inst. of Standards & Technol., Vol. 99, No. 5, October 1994, pp. 658–660, for these reasons up to the present time only two pitch magnification references have been certified to be used as SEM magnification standards. They have the required characteristics and have been created with nominal pitch values less than 1 micron [NIST Standard Reference Materials SRM484 and SRM-2090]. Such magnification references are unique, expensive and not readily available for most users.
In particular, pitch magnification reference SRM-2090 contains 8 separate parallel line structures, the pitch value between adjacent line features is about 200 nanometers. It is known that optical diffraction methods can be used to provide highly precise and accurate pitch certifications of a diffraction grating. However, one is not able to apply these techniques to the certification of NIST SRM-484 and SRM-2090 because of the small number of repeated lines. For these purposes the authors U.S. Pat. No. 5,822,875 had to provide a special unique measuring environment that protected the SEM from vibrations and contained a precision stage with a laser inteiferometer, operating under computer control. The achievable accuracy of measurement certification is not given by the authors [U.S. Pat. No. 5,822,875].
Previous inventions attempted to improve the accuracy of SEM measurements, but had a number of drawbacks. A major issues that these inventions failed to provide a universal solution for accurate magnification calibration of image magnifying instruments. More specifically, the invention described in the patent “Scanning electron microscope ruler and method”, U.S. Pat. No. 5,822,875, is restrictive because it requires fabrication of precision features on the integrated circuits to be inspected. This precludes the method from being readily available to a wide number of microscope users. More importantly, the ruler method described in the patent U.S. Pat. No. 5,822,875 did not provide for the highest degree of precision and accuracy. This degree is achievable in our proposed invention.
In the patent, “Apparatus and method for measuring length in scanning particle microscope” U.S. Pat. No. 4,677,296, the propose measuring references provided an inferior degree of magnification calibration because they cannot be used as an absolute magnification standard. In addition they do not incorporate the necessary algorithms to determine the statistical measurement errors. Thus, the degree of accuracy of the magnification measurement is unknown. In contrast, our proposed invention enables the user to precisely determine the magnification errors in the microscope.
Finally, the invention described in patent, “High precision calibration and feature measurement system for a scanning probe microscope” U.S. Pat. No. 5,825,670 is measuring instrument specific, and has limited use because it requires direct control of the SEM‘s’ scan drive circuitry. Also, it cannot be applied to non-scanning image magnifying instruments. In contrast, our proposed invention can be universally applied to any image magnifying device.